TECLAB   Shaw. Din   shaw@teclab.cn

Summary:This article mainly introduces two optimized non-destructive testing methods for detecting additive manufacturing (AM) parts: based on ultrasonic resonance spectroscopy RUS and fully focused phased array ultrasonic PAUT (using FMC full matrix acquisition and TFM fully focused imaging). According to the ISO/TC261-ASTM/F42 international standard, two testing methods are used to test star shaped parts manufactured using additive manufacturing. The parts contain typical additive manufacturing delamination defects of 100-700 μ m. The RUS method can identify whether parts contain defects, but it cannot determine the quantity, location, and size of defects. The detection effect of PUAT depends on the shape, direction, and position of the defect. The best detection effect is for vertical cylindrical defects made for simulating layered defects, with a detection accuracy of up to 200 μ m

keyword:Non destructive testing, volumetric non-destructive testing, additive manufacturing, ultrasonic resonance testing, phased array ultrasonic testing, fully focused imaging

introduce

With the application and development of metal additive manufacturing technology in the industrial field, especially in the aerospace and medical fields, more and more critical spare parts are produced using metal additive manufacturing technology. Due to the complex shapes and unsatisfactory surface roughness of the parts produced by additive manufacturing, traditional detection methods are often no longer applicable. In order to ensure the quality of products produced by this technology, it is urgent to seek a non-destructive and rapid testing method.

For volumetric testing, X-ray based computer imaging technology XCT is often the most suitable detection method, and the shape and surface roughness of the tested object are often not limited. The size and density of the tested object are then limited by the emission power of the X-ray source and the detection chamber. Therefore, XCT is often a highly expensive and time-consuming detection method.

RUS ultrasonic resonance testing technology does not detect the specific details of the parts, but identifies whether the parts meet quality requirements by analyzing the resonance frequency of the parts. The operation is simple, fast, and not affected by the size, shape, and roughness of the parts.

PAUT phased array ultrasonic testing technology is also an optional testing method, which can detect some shape responsible parts without moving the probe through ultrasonic beam deflection. And it can construct three-dimensional images to define the type and location of defects.

Additive Manufacturing Sample

According to the guidance instructions of the additive manufacturing standard ISO/TC261-ASTM/F42, the PBF laser powder bed melting technology is used to 3D print stainless steel star shaped samples as follows: the printing chamber size is 0.11 mm, the printing layer thickness accuracy is 40 um, and the printing speed is 755.5 mm/s

Figure 1. Metal 3D printer (supporting stainless steel, tool steel, Inconel, and copper), cleaning machine, sintering furnace, and star shaped additive manufacturing samples

Design S0 defect free, S1 full-size defect specimen, and S2 half size defect specimen, with artificial defects located in critical areas and difficult to reach locations of the specimens. Marked as R1-R5 respectively. As follows:

1. Cross layer defects are simulated by printing vertical cylindrical space defects of the same length with different diameters. The cylindrical space defects are connected and have external openings for easy discharge of metal powder;

2. Layered defects are simulated by printing horizontal cylinders of the same length with different diameters, and the same external opening facilitates the discharge of metal powder;

3. Loose defects are simulated by printing spherical spaces with different diameters and cylindrical spaces with different directions inside.

All defect diameters range from 100 μ m to 700 μ m. Full size specimen parameters: h=45 mm, a=60 mm,

Figure 2. Defect diagram of the sample

RUS ultrasonic resonance detection method

RUS is a comprehensive comparative testing method that establishes testing standards by measuring the resonance frequency of defect free samples, and then determines whether the sample quality is qualified or unqualified by comparing the measured resonance frequency of the samples. There are usually two ways to induce resonance in the sample: RAM acoustic resonance method and RUS ultrasonic resonance method. RAM usually uses external tapping such as a small hammer to induce resonance in the sample, and then amplifies the resonance frequency signal of the sample through sensors or microphones for resonance spectrum comparison. RUS uses an ultrasonic transducer for frequency scanning excitation, and then receives resonance frequency signals through the ultrasonic transducer to obtain resonance spectra of different frequency bands for comparative analysis.

Figure 3. RUS ultrasonic resonance detection system and RAM acoustic resonance detector

A total of 88 samples were tested, including 40 S0 intact samples. After establishing a complete product resonance spectrum database, different defect S1 and S2 samples were tested. The resonance frequency range of the collected samples is from 500 Hz to 50 KHz. After measurement, RAM can effectively distinguish all defective samples and the vast majority of intact tests (one of which failed to distinguish intact samples)

Figure 4. Resonance spectrum of sample RAM detection

PAUT-FMC/TFM phased array ultrasonic testing method

The S1 and S2 samples were tested using ultrasonic phased array full matrix and fully focused imaging techniques. The sample is soaked in a water tank and B-scan and C-scan images are obtained through scanning.

Figure 5. PIONEER 128/128 Fully Focused Phased Array Imaging Detection System

Figure 6. Parameters of the ultrasonic phased array probe used in the experiment: linear array, 10 MHz, 0.25 mm element spacing

Figure 7. Principle of Full Matrix Acquisition: Each chip emits, all chips receive, and 128 channel chips are sequentially excited and received to obtain 128x128 A-wave signals

Through testing, it was found that:

In sample S1:

Four cylindrical defects in different directions (diameter 0.3 mm, length 2 mm) can be detected - Figure 8,

Among the six vertical cylindrical defects, defects with diameters ranging from 200 um to 700 um can be seen, while defects with diameters of 100 um and 150 um cannot be detected - Figure 9.

Four spherical defects with diameters ranging from 400 μ m to 700 μ m can be detected - Figure 10,

Horizontal cylindrical defects cannot detect effective signals.

In sample S2:

Three internal - Figure 11 and four external - Figure 12 horizontal cylindrical defects can be detected

Vertical cylindrical defects with diameters ranging from 200 um to 700 um can be detected, while those with diameters of 100 um and 150 um cannot be detected - Figure 13,

Only a cylindrical defect in a different direction can be detected - Figure 14, spherical defects cannot be detected.

Figure 8. S1 sample, simulated porous closed cylinder defect imaging in different directions D 0.3 x L 2 mm

Figure 9. Imaging of vertical cylindrical defects with diameters ranging from 200 μ m to 700 μ m for S1 specimen

Figure 10. Imaging of four spherical defects with diameters ranging from 400 um to 700 um in sample S1

Figure 11. S2 sample, imaging of internal horizontal cylindrical defects

Figure 12. S2 sample, imaging of external horizontal cylindrical defects

Figure 13. S2 specimen, vertical cylindrical defect imaging with diameters ranging from 200 um to 700 um

Figure 14. S2 sample, imaging of cylindrical defects in different directions

summary

The RAM acoustic resonance method can achieve fast and non-destructive identification of the quality of additive manufacturing parts, but it cannot determine the type and location of defects. The PAUT phased array ultrasonic testing method can determine the location and type of defects, but defects that are too small cannot be detected, and the direction of the defects also affects the method. Summarize the advantages and disadvantages of the two methods as follows:

Advantages of RAM:

• Easy to use

• Quickly

• Objective

• Not limited by the shape (especially grid structure) and size of the workpiece

No need for infiltration, no need for coupling agents, etc

• Can identify products with quality issues

Disadvantages of RAM:

• Overall inspection, detecting NG or OK parts

• Unable to identify defect type and location

Advantages of PAUT:

• Scannable and imageable

• It can also deflect the sound beam without moving the probe for scanning

• Faster than conventional ultrasound

The limitation on the shape of the workpiece is lower than that of conventional ultrasound

• Limit the size of the workpiece to below XCT

• Identifiable defect types and locations

• Better spatial resolution than conventional ultrasound

Disadvantages of PAUT:

Compared to conventional ultrasound and RAM, operators require more experience and training

• Less efficient than RAM detection

• Restrictions on the shape and size of the workpiece

• Requirements for surface roughness

• Requires coupling agent

reference:

“Efficient volumetric non-destructive testing methods for additively manufactured parts”， International Institute of Welding 2020，https://doi.org/10.1007/s40194-020-00932-0

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